A gravity gradiometer is disclosed in our International Patent Application No. PCT/AU2006/001269 and several concurrently filed applications. The content of International Patent Application No. PCT/AU2006/001269 is incorporated into this specification by this reference.
Gravimeters are widely used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.
Typically such devices have been used to attempt to locate deposits such as ore deposits including iron ore and geological structures bearing hydrocarbons.
The above-mentioned gradiometer has a sensor in the form of a sensor mass which is pivotally mounted for movement in response to the gravity gradient.
Gravity gradiometers of the type described above are typically mounted in an aircraft and carried by the aircraft while making measurements. The consequence of this is that the gradiometer can move with movements of the aeroplane. This creates accelerations of the gradiometer which are detected by the instrument and if not compensated for, will produce noise or swamp actual accelerations or movement of the gradiometer in response to the gravity gradient which is to be detected by the gradiometer. The unwanted accelerated can be compensated for by signal compensation which couples accelerations into the sensing circuitry. However, these signals may not get into the SQUID device which makes the actual measurements of the gravity gradient incorrect. Therefore, some attempts have been made to couple signals indicative of the unwanted acceleration directly into the SQUID device. This results in the frequency response of the accelerometer needing to be matched to the SQUID device. However, this technique often only works in relation to DC currents and not at higher frequency currents. Another technique is a force compensation which uses linear accelerometers to move the sensor masses to compensate for the unwanted acceleration. This technique therefore requires the sensing equipment to distinguish between angular motions which are generally wanted and linear motions of the masses which are the motions which cause the unwanted acceleration. Signals indicative of the linear acceleration are therefore applied to an actuator to move the bars to cancel out the effect of the unwanted acceleration.
The purpose of both of the above techniques is to ensure that the spacing between the masses and transducers at ends of the masses are the same when measurements commence, so that any angular movement of the bar about a pivot point can be properly detected because that movement will move the bar closer to one of the transducers and further away from another of the transducers by the same amount, thereby enabling the angular movement of the bar in response to the gravity gradient to be properly detected.